A thermal processing chamber as used herein refers to a device that rapidly heats objects, such as semiconductor wafers. Such devices typically include a substrate holder for holding a semiconductor wafer and a light source that emits light energy for heating the wafer. During heat treatment, the semiconductor wafers are heated under controlled conditions according to a preset temperature regime. During heating, various processes can be carried out within the thermal processing chamber, such as rapid thermal oxidation, nitridation, annealing and silicidation.
Many semiconductor heating processes require a wafer to be heated to high temperatures so that the various chemical and physical transformations can take place as the wafer is fabricated into a device. During rapid thermal processing, for instance, semiconductor wafers are typically heated by an array of lights to temperatures from about 400.degree. C. to about 1,200.degree. C., for times which are typically less than a few minutes. During these processes, one main goal is to heat the wafers as uniformly as possible.
During the rapid thermal processing of a semiconductor wafer, it is desirable to monitor and control the wafer temperature. In particular, for all of the high temperature wafer processes of current and foreseeable interest, it is important that the true temperature of the wafer be determined with high accuracy, repeatability and speed. The ability to accurately measure the temperature of a wafer has a direct payoff in the quality and size of the manufactured integrated circuit. For instance, the smallest feature size required for a given semiconductor device limits the computing speed of the finished microchip. The feature size in turn is linked to the ability to measure and control the temperature of the device during processing.
In the past, the temperature of semiconductor wafers has been monitored during heat treatment using radiation sensing devices, such as pyrometers, that sense the radiation being emitted by the semiconductor wafer at a selected wavelength. By sensing the thermal radiation being emitted by the wafer, the temperature of the wafer can be calculated with reasonable accuracy. Pyrometers, however, measure an apparent temperature of an object instead of its true temperature. In particular, the temperature of an object sensed by a pyrometer is dependent upon the object being opaque and upon the object's emissivity, which is rarely known for semiconductor wafers.
Thus, in order to measure the true temperature of a semiconductor wafer during heat treatment using a pyrometer, the indicated temperature must be corrected to account for the emissivity. Unfortunately, the emissivity of a semiconductor wafer is generally unknown and is very difficult to measure accurately. The emissivity of semiconductor wafers, which varies from wafer to wafer, is a property of the surface and depends on several parameters, such as the chemical composition of the wafer, the thickness of the wafer, the surface roughness of the wafer, the temperature of the wafer, and the wavelength at which the pyrometer operates. Further, at lower temperatures, semiconductor wafers can be partially transparent thus causing the emissivity of the wafer to vary. Consequently, one major drawback to measuring the temperature of semiconductor wafers using pyrometers is that the pyrometers cannot accurately determine the temperature of the wafers at lower temperatures, such as below about 500.degree. C.
Besides using pyrometers, it has also been proposed in the past to use thermocouples for monitoring the temperature of the wafers. Thermocouples generally measure the true temperature of objects. In order for thermocouples to measure the temperature of an object, however, the thermocouple typically has to be in contact with the object, which presents a number of disadvantages. For instance, when in contact with a wafer being heated, a thermocouple can create temperature discontinuities throughout the wafer. Attaching a thermocouple to a wafer also makes it more difficult to rotate the wafer during processing. Rotating the wafer during heat treatment is generally preferred in order to enhance temperature uniformity and promote uniform contact between the wafer and any gases contained within the chamber. Having to place a thermocouple in contact with a wafer being heated also can make it more difficult to load and unload wafers from the chamber since the wafer has to be properly aligned with the thermocouple prior to being heated.
In view of the above, a need currently exists for a system and method of measuring the temperature of semiconductor wafers during thermal processing applications, especially when the wafers are at lower temperatures. A need also exists for a system for measuring the temperature of semiconductor wafers using thermocouples wherein the thermocouples do not have to be placed in contact with the wafers.